1. Field of the Invention
The present invention relates to a method of producing a cordierite honeycomb structure. More specifically, the present invention relates to a method of producing a cordierite honeycomb structure capable of enhancing a strength of a dried honeycomb product and suppressing formation of cracks during firing.
2. Description of Related Art
A need for removing fine particles or toxic substances in an exhaust gas from an internal combustion engine or a boiler has increased in consideration of its adverse effects on the environment. In particular, regulations regarding removal of fine particles (hereinafter, may be referred to as “PM”) discharged from a diesel engine tend to tighten worldwide. Use of a filter (honeycomb filter) comprising a honeycomb structure as a trapping filter (diesel particulate filter, hereinafter, may be referred to as “DPF”) for removing PM has attracted attention, and various systems have been proposed. The DPF generally has a structure obtained by: forming a plurality of cells serving as fluid passages and partitioned by porous partition walls; and alternately plugging the cells. Thus, the porous partition walls constituting the cells serve as filters. Further, cordierite having a small coefficient of thermal expansion and high thermal shock resistance is preferably used as a material for the DPF.
The honeycomb filer comprising such a honeycomb structure is installed in an exhaust gas system of a diesel engine and is used. The diesel engine is installed on a heavy vehicle such as a truck or a bus and has a large displacement. Thus, the honeycomb structure must be increased in size and reduced in wall thickness. However, production of a honeycomb structure increased in size and reduced in wall thickness has a problem of formation of cracks in partition walls during firing.
Meanwhile, there is disclosed a method of producing a honeycomb filter of silicon carbide, not of cordierite, using raw material powder partially having a circularity of 0.85 or less for enhancing a porosity of the honeycomb filter while suppressing warping thereof during production and maintaining a bending strength thereof (see JP 2002-293660 A, for example). However, formation of cracks is not necessarily suppressed during production by the method of producing a cordierite honeycomb structure increased in size and/or reduced in wall thickness even if the raw material has a circularity of 0.85 or less.